In physics is called adiabatic a process or physical transformation of macroscopic variables of a thermodynamic system (pressure, temperature, volume) from one physical state to another without the transfer of heat or mass of substances between a thermodynamic system and the environment surrounding the system. Of it are possible the adiabatic expansion in which the thermodynamic system performs work at the expense of the internal energy of the system with consequent cooling (for example a mass of air rising in the atmosphere) and the adiabatic compression that represents the opposite process and that brings therefore to the increase of the internal energy and therefore to the heating of the system (for example the compression of a mass of air falling in the atmosphere for effect of the atmospheric subsidence).
In an adiabatic process, energy is transferred to the surroundings only as work. The adiabatic process provides a rigorous conceptual basis for the theory used to expound the first law of thermodynamics, and as such, it is a key concept in thermodynamics.
This is a type of thermodynamic process only ideally realizable; however many processes can be treated with good approximation as adiabatic since the heat exchange with the outside can be considered negligible. A sufficient condition for heat exchange with the outside world to be considered negligible is that the thermal time constant of the system hosting the process is much greater than the duration of the process itself.
Adiabatic processes can generally be considered to be processes that:
- occur very quickly (e.g., the expansion of combustion gases within the pistons of an endothermic engine);
- occur within systems that are very well isolated from the surrounding environment (e.g. in refractory containers or under vacuum);
- occur within systems with a high thermal capacity (e.g. in systems where the high characteristic dimension and the high specific heat make the heat exchanged through the surface negligible compared to the heat accumulated in the volume of the system);
- occur at a temperature not very different from the temperature of the surrounding environment.
By extension, it is used in gasdynamics to denote gas currents in which there is no energy exchange with the external environment (adiabatic flow).
In mechanics, the term refers to systems where the change in forces and constraints acting on them occurs very slowly with respect to certain time parameters. For example, the adiabatic pendulum is a pendulum in which the short length varies very slowly with respect to the period of oscillation.
In meteorology, adiabatic atmosphere means the state of hydrostatic equilibrium especially of the lower layers of the atmosphere in which there is no heat transfer from the ground. Adiabatic transformations are those that cause air temperature to decrease with altitude in the troposphere by virtue of air masses rising by convection, and in areas of high pressure the weather is predominantly stable and sunny. In such a condition, for every 100 meters of altitude, the air temperature decreases by about 1 ºC if the air is dry, or by 0.6 ºC if the air is moist.
In atomic physics is important the Ehrenfest principle on adiabatic, formulated as follows: if a system is in a quantum state, also the state to which the system is brought after an adiabatic transformation is a quantum state and has the same quantum numbers.